检查python版本：
python --version
Python 2.7.12
方法一 （推荐）
1、检查Python的替代版本是否被update-alternatives 命令识别
root@JoJo:~# update-alternatives --list python
update-alternatives: error: no alternatives for python
报错表示没有
2、更新替换列表
root@JoJo:~# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode
root@JoJo:~# update-alternatives --install /usr/bin/python python /usr/bin/python3.5 2
update-alternatives: using /usr/bin/python3.5 to provide /usr/bin/python (python) in auto mode
查看：
root@JoJo:~# update-alternatives --list python
/usr/bin/python2.7
/usr/bin/python3.5
3、选择版本
root@JoJo:~# update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 2 auto mode
1 /usr/bin/python2.7 1 manual mode
2 /usr/bin/python3.5 2 manual mode
Press <enter> to keep the current choice[*], or type selection number: 0
检查版本号：
root@JoJo:~# python --version
Python 3.5.2
替换成功
4、删除旧版本 （可选）
update-alternatives --remove python /usr/bin/python2.7
不推荐直接删除软链接，反正update-alternatives可以配置...就别删了
方法二
1、检查系统是否默认安装了3.5
root@JoJo:~# whereis python3.5
python3: /usr/bin/python3.5 /usr/bin/python3.5m /usr/bin/python3 /usr/lib/python3.5 /usr/lib/python3 /etc/python3.5 /etc/python3 /usr/local/lib/python3.5 /usr/share/python3 /usr/share/man/man1/python3.1.gz
已安装
也可用：
root@JoJo:~# ls /usr/bin/python*
/usr/bin/python /usr/bin/python2.7 /usr/bin/python2-config /usr/bin/python3.5 /usr/bin/python3m
/usr/bin/python2 /usr/bin/python2.7-config /usr/bin/python3 /usr/bin/python3.5m /usr/bin/python-config
2、删除2.7的Python链接文件
rm -rf /usr/bin/python
3、新建指向Python3.5的软链接
ln -s /usr/bin/python3.5 /usr/bin/python
4、添加环境变量
PATH=/usr/bin:$PATH
替换完成
